1. Field of the Invention
The present invention relates to a process of coating a molded article of an olefinic resin. More particularly, it relates to a process of coating a molded article of an olefinic resin which process is capable of forming a finishing coating layer in a single coating process without washing the surface of the molded article and without coating the surface of the molded article with a primer as undercoat.
2. Description of the Related Arts
For automobile bumpers, protective strips and the like, compositions containing polypropylene (PP) and ethylene-propylene rubber (EPR) have heretofore been used as materials. Polypropylene has very inferior property for coating because it has no polar group in the molecule and is chemically inert. For coating of a bumper made of such a composition, degreasing and etching treatment with trichloroethane or the like is indispensable. This situation is not limited to coating of automobile bumpers. For coating of an article made of a polypropylenic composite material, it is the general practice that the article is processed with pretreatment of etching with trichloroethane to obtain practical coating strength (refer to Shosaku Yamamoto and Mikio Uchida, Jitsumu Hyomen Gijutsu, Vol. 31, Number 10, Page 440 (1984)).
However, trichloroethane causes environmental destruction, such as ozonosphere destruction., and thus, a method using trichloroethane in coating must be avoided.
As materials to replace trichloroethane, surface active agents in the form of alkaline aqueous solutions can be mentioned. However, these surface active agents show low washing and dissolving ability when they are applied to PP and cannot be used for coating of an article made of a polypropylenic composite material, such as the composition described above and the like.
Because of the situation described above, the coating property of molded articles of olefinic resins like polypropylene has been improved both with respect to the coating material and with respect to the resin composition.
As the improvement with respect to the coating material, resins or coating materials having good adhesion to the surface of molded articles of olefinic resins are disclosed. Examples of such resins and coating materials are a mixture of a chlorinated olefinic resin and acrylic resin (Japanese Patent Application Laid-Open No. Showa 62(1987)-200438), a resin system in which an isocyanate compound is used in combination with a copolymer of a chlorinated olefinic resin and acrylic monomer (Japanese Patent Application Laid-Open No. Showa 62(1987)-27968), and the like.
On the other hand, as the improvement with respect to the resin composition, methods of improving the coating property by blending polar substances with PP have been reported. Examples of such methods are a method utilizing a composition prepared by adding polyvinylphenol to a PP resin (Japanese Patent Application Laid-Open No. Heisei 3(1991)9941), a method utilizing a composition prepared by adding a copolymer of ethylene and carboxylic acid compound to a PP resin (Japanese Patent Application Laid-Open No. Heisei 3(1991)-278862), and the like.
However, these coating materials and compositions of olefinic resins do not always have sufficient adhesion of the coated layer when molded articles prepared by using these materials are coated without washing the surface or after washing the surface with a watery system. Stability, solvent resistance, weatherability, and the like of the coating material are not satisfactory, either. These materials have another drawback that the total cost is increased because a new kind of washing process or an expensive primer specific for the material is required.
Furthermore, application of these disclosed technologies are all limited to articles which do not require coating with higher adhesion or good weatherability, such as automobile interior parts, home electric appliances, and the like. These technologies cannot realize coating of automobile exterior parts, such as a bumper and the like, in a single coating process without washing of the surface and without coating of the surface with a primer as undercoat.
Therefore, it has been desired by the automobile industry that a coating material or an olefinic resin composition which enables formation of a coating layer on a substrate directly without washing and finishing of the coating in a single process or within several processes be developed.